1. Field of the Disclosure
The present disclosure relates generally to optical imaging systems. The present disclosure relates more particularly to active optical fibers, amplified spontaneous emission (ASE) sources using such active optical fibers, and imaging and detection systems and methods using such ASE sources.
2. Technical Background
Conventional amplified spontaneous emission (ASE) based light sources combine broadband emission, similar to a light emitting diode (LED), with high spatial coherence and high power per mode, similar to a laser. Examples include, for example, fiber-based ASE sources and semiconductor-based superluminescent diodes. See, e.g., P. Wang et al., “110 W double-ended ytterbium-doped fiber superfluorescent source with M2=1.6,” Opt. Lett 31, 3116-3118 (2006); M. Rossetti et al., “Superluminescent light emitting diodes: the best out of two worlds,” Proc. SPIE 8252, 825208 (2012). ASE sources have become increasingly popular for a range of applications including spectroscopy, optical coherence tomography (OCT), fiber sensors, and gyroscopes. See, e.g., W. Denzer et al., “Near-infrared broad-band cavity enhanced absorption spectroscopy using a superluminescent light emitting diode,” Analyst 134, 2220-2223 (2009); A. F. Fercher et al., “Optical coherence tomography—principles and applications,” Reports Prog. Phys. 66, 239-303 (2003); H. S. Choi et al., “High-performance fiber-optic temperature sensor using low-coherence interferometry,” Opt. Lett. 22, 1814-1816 (1997); B. Lee et al., “Review of the present status of optical fiber sensors,” Opt. Fiber Technol. 9, 57-79 (2003). However, the high spatial coherence of existing ASE sources has precluded their use in full-field imaging applications, where spatial coherence introduces undesirable artifacts such as speckle. By comparison, traditional low spatial coherence sources such as thermal sources and LEDs do not provide the required power per mode for high speed, full-field imaging applications. See B. Karamata et al., “Multiple scattering in optical coherence tomography. II. Experimental and theoretical investigation of cross talk in wide-field optical coherence tomography.,” J. Opt. Soc. Am. A. 22, 1380-1388 (2005). Recently, there have been several demonstrations of multimode lasers which combine low spatial coherence with high power per mode, including dye-based random lasers, powder-based random Raman lasers, solid-state degenerate lasers, semiconductor-based chaotic microcavity lasers, and semiconductor-based large-area VCSELs and VCSEL arrays. See, e.g., B. Redding et al., “Speckle-free laser imaging using random laser illumination,” Nat. Photonics 6, 355-359 (2012); A. Mermillod-Blondin et al., “Time-resolved microscopy with random lasers,” Opt. Lett. 38, 4112-4115 (2013); B. H. Hokr et al., “A narrow-band speckle-free light source via random Raman lasing,” J. Mod. Optics, 63, 46-49 (2015); M. Nixon et al., “Efficient method for controlling the spatial coherence of a laser.,” Opt. Lett. 38, 3858-3861 (2013); R. Chriki et al., “Manipulating the spatial coherence of a laser source,” Opt. Express 23, 12989-12997 (2015); B. Redding et al., “Low spatial coherence electrically pumped semiconductor laser for speckle-free full-field imaging,” Proc. Natl. Acad. Sci. 112, 1304-1309 (2015); F. Riechert et al., “Speckle characteristics of a broad-area VCSEL in the incoherent emission regime,” Opt. Commun. 281, 4424-4431 (2008); G. Craggs et al., “Thermally controlled onset of spatially incoherent emission in a broad-area vertical-cavity surface-emitting laser,” IEEE J. Sel. Top. Quantum Electron. 15, 555-562 (2009); G. Verschaffelt et al., “Spatially resolved characterization of the coherence area in the incoherent emission regime of a broad-area vertical-cavity surface-emitting laser,” IEEE J. Quantum Electron. 45, 249-255 (2009); J.-F. Seurin et al., “Progress in high-power high-efficiency VCSEL arrays,” Proc. SPIE 7229, 722903 (2009). However, an optical fiber based light source with low spatial coherence has not been demonstrated. In addition, each of these previous demonstrations of low spatial coherence lasers provided narrow bandwidth emission with relatively high temporal coherence, precluding their use in ranging applications such as OCT or frequency resolved LiDAR. See, e.g., W. Drexler et al., Optical Coherence Tomography (Springer-Verlag, Berlin Heidelberg, 2008); W. C. Swann et al., “Frequency-resolved range/doppler coherent LIDAR with a femtosecond fiber laser,” Optics Letters 31, 826-828 (2006).